As a conventional planar antenna device, a thin, lightweight microstrip patch antenna is widely available (For example, see Patent Reference 1). The patch antenna has a structure such that a ground conductor (ground) and a rectangular antenna conductor (also known as an “antenna element”) to serve as an antenna unit are formed on the back and the face side of a dielectric board, respectively. As electrical power is fed to the antenna conductor, radio waves are radiated at a frequency where resonance occurs according to the length of the longer sides of the antenna conductor. In order to feed electrical power to the antenna conductor, the coplanar feeding system is available in which a feed line provided as a microstrip line (also known as a “feed line”) is formed on the surface of the dielectric board, the surface where the antenna conductor is also formed. In this system, which forms the antenna conductor and the feed line on the same surface, a planar antenna device can be manufactured with ease at a low cost.
Typically, the antenna conductor and the feed line are connected at a position called a “feeding point”. The design is made such that the impedances of the antenna conductor and the feed line are matched at the feeding point. By matching the impedances so as to prevent reflections from occurring, electrical power can be fed to the antenna conductor with efficiency.
Here, a brief explanation is given as to how to match the impedances of the antenna conductor and the feed line. The impedance of the antenna conductor varies according to its position on the antenna conductor. The impedance is low around the central portion in the antenna, becomes higher toward the end portion, and reaches a value almost equal to infinity at the end. Therefore, a cut (also called as a “matching slit”) is made in the antenna conductor down to position of the feeding point, at which the antenna conductor and the feed line are connected, so that the antenna conductor has the same impedance as the feed line. The method thus matches the antenna conductor and the feed line.
In order to use the planar antenna device practically, a predetermined radiation pattern or radiant gain is required. A radiation pattern and a radiant gain characteristic depend on the entire effective aperture dimensions of the antenna conductor. With an antenna conductor of larger dimensions, the antenna's directivity increases and a higher radiant gain is obtained. In the case where the patch antenna is employed singly, since the size of the antenna conductor is determined by the frequency to be used, the radiation directivity decreases and the gain is low. Therefore, in order to adjust a radiation pattern and a radiant gain, the array structure is employed, in which a plurality of antenna conductors are arranged at a specific regular spacing, so as to adjust effective aperture dimensions. However, as the interval between the antenna conductors becomes wider, the side lobe of the radio waves radiated from the array antenna increases in level, the feed line has to be installed into the narrow interval limited by the antenna conductors.
Here, a description is given for the structure of a conventional planar array antenna employing the coplanar feeding system with reference to FIG. 1. FIG. 1 is a top view showing a conventional planar array antenna 1000. The planar array antenna 1000 shown in FIG. 1 includes a dielectric board 1010, four antenna elements 1001, a feed line 1002, and a ground conductor (not shown). Each of the antenna elements 1001 is connected to the power feeder 1005 via the feed line 1002 (in this case, the point where each of the antenna elements 1001 and the feed line 1002 are connected together, is referred to as a “feeding point”). Each of the antenna elements 1001 has a matching slit 1003 for matching the impedances of the antenna elements 1001 and the feed line 1002. The ground conductor is provided at the back of the planar array antenna 1000.
Generally, the array structure shown in FIG. 1, where the antenna elements 1001 are connected to the power feeder 1005 in parallel (tree-shaped structured connection), is available for radiation over a wide range of frequencies.
In the planar array antenna 1000, the antenna elements 1001 have to be excited in phase. This is because the radio waves radiated from the antenna elements 1001 cancel each other thereby degrading their function as an array antenna, in the case where the radio waves from the antenna elements 1001 are out of phase each other.
The planar array antenna 1000, therefore, is designed so that the electrical lengths from the power feeder 1005 to the respective antenna elements 1001, or equivalently, the lengths of the respective sections of the feed line 1002 are equal. Furthermore, in order to excite the electric fields of the respective antenna elements 1001 in a same direction, the respective sections of the feed line 1002, the respective feeding points 1004, and the respective antenna elements 1001 are arranged in a same way. Specifically, the structure is such that the respective sections of the feed line 1002 connect to the respective antenna elements 1001 at the respective feeding points 1004 from a same side.
Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2004-166043